Add a variable to track whether or not we've used a unique section,

e.g. linkonce, to TargetMachine and set it when we've done so
for ELF targets currently. This involved making TargetMachine
non-const in a TLOF use and propagating that change around - I'm
open to other ideas.

This will be used in a future commit to handle emitting debug
information with ranges.

llvm-svn: 199871